This invention relates to electroluminescent phosphors and more particularly to such phosphors encapsulated with a life-extending coating. Still more particularly, it relates to a method of recovering the base electroluminescent phosphors from such encapsulated materials.
Phosphors are a known class of materials that emit electromagnetic radiation, usually in the visible region of the spectrum, upon excitation by another form of energy. Phosphors are usually named for the type of energy to which they susceptible. For example, cathodoluminescent phosphors are excited to emission by impingement of electrons or cathode rays; photoluminescent phosphors are excited to emission by impingement of actinic radiation; x-ray phosphors by the impingement of x-rays; and electroluminescent phosphors by the action of an electric field, usually supplied by an alternating current. It is with the latter type of phosphor that this invention is most particularly concerned.
Commercial electroluminescent phosphors are mostly based upon a host material of zinc sulfide activated by one or more activators among which are copper and/or manganese and/or chloride, which are incorporated into the host structure. These phosphors have uses as lamps or illuminating sources for readout devices or watch faces. Two basic kinds are employed, one in which the phosphor is dispersed in an organic binder and laminated in plastic, and a second variety in which the phosphor is dispersed in a ceramic dielectric and fused to a metal plate.
The plastic variety is the more economical; however, the light output of the phosphor encased in an economical plastic such as Mylar(trademark) degrades rapidly, such phosphors having a half-life of about 50 hours at operation at 400 Hz and 100 volts. (The half-life of a phosphor is that point in time when its light output is reduced by half from some initial light output). Incorporation of the phosphor in a more expensive plastic lamina such as Aclar(trademark) will produce a lamp having much better life (for example, a half-life of 500 to 2000 hours at operating levels of 400 Hz and 100 volts) with, however, a concomitant increase in the cost of the lamp.
To further increase the life of the electroluminescent phosphors it is known to encapsulate individual phosphor particles with a continuous, conformal dielectric, moisture-inhibiting material that allows the use of the phosphor in the less expensive plastic materials. For example, U.S. Pat. No. 6,064,150 teaches an electroluminescent phosphor having thereon a coating of aluminum nitride or aluminum nitride amine. Other coatings have included aluminum oxide hydroxide. An instance of the latter is disclosed in Ser. No. 09/153,978, filed Sep. 16, 1998, now abandoned, and assigned to the assignee of this invention.
It would be an advance in the art if a method could be provided for recovering a base electroluminescent phosphor from an encapsulated material.
It is, therefore, an object of the invention to provide a method of recovering a base phosphor from an encapsulated one.
It is another object of the invention to provide an economical process for recovery.
These objects are accomplished, in one aspect of the invention, by a method of recovering a base phosphor from encapsulated phosphors, which comprises the steps of selecting a recovery vessel including a magnetic stir bar; adding to the recovery vessel, deionized water, a first batch of concentrated hydrochloric acid and encapsulated phosphor to form a stirred suspension; heating the stirred suspension to about 90xc2x0 C. for about 2 hours; settling the stirred suspension; decanting the mother liquor and slurrying the remaining solids with a second batch of hydrochloric acid and water; decanting and washing the remaining solids with deionized water; filtering the remaining solids to form a filter cake; heating the filter cake at 110xc2x0 C. in air for a time sufficient to form a dried filter cake; and screening the dried filter cake through a xe2x88x92325 mesh screen to yield a recovered base phosphor.
The process is economical and yields a base phosphor with good performance characteristics.
For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof; reference is made to the following disclosure and appended claims.
Base phosphors were recovered from encapsulated phosphors coated by two different processes leading to coatings of two different materials. The first material is designated in Tables I and II as ANE. This material is a zinc sulfide-based electroluminescent phosphor coated with aluminum nitride amine by the process disclosed in the above-cited U.S. Pat. No. 6,064,150. The second material is designated in Table III as TNE. This material also is a zinc sulfide-based electroluminescent phosphor, coated, however, with aluminum oxide hydroxide by the process disclosed in the above-cited co-pending application, Ser. No. 09/153,978.